Strong pulsed radio frequency (RF) fields in the imaging cavity of an MRI machine may induce very high RF currents in the conventional long, low resistance, electrical cables typically used with standard diagnostic electronic instruments, such as temperature measuring devices. These cables and the associated sensors can become very hot due to RF I.sup.2 R heating, and if in contact with a patient's skin, can cause severe burns. Apart from the possibility of burning the patient, RF heating of a temperature sensor would nullify its usefulness as a temperature measuring device. In addition, the RF pulses, which are picked up on the long, low resistance, electrical cables normally used when measuring temperature, often enter the data monitoring equipment where they can degrade measurement accuracy. Conversely, many data monitoring instruments generate high frequency noise signals. These signals are often radiated from the instrument cables in the form of electromagnetic waves and are picked up by the MRI machine, which can cause artifacts in the MRI image.
U.S. Pat. No. 5,730,134 to Dumoulin et al. ("the '134 Patent") discloses a temperature monitoring system which employs a temperature sensor that is incorporated into a device 150 that is inserted into a body during an MRI scan. The inserted device is used in conjunction with the MRI machine to position itself within the body or for localized magnetic resonance imaging within the body. As shown in FIG. 2, the inserted device 150 comprises a a small RF coil 200 which is electrically coupled to the MRI system via conductors 210 and 220. The temperature sensor used to monitor the temperature of inserted device 150 could undergo localized heating due to induced RF currents. If a temperature rise in excess of a predetermined threshold is detected, power to the MRI machine is either reduced or cut off. In the preferred embodiment shown in FIG. 2, the temperature sensor consists of a flouresescent substance 275 located at the remote end of fiber optic cable 270, which is connected to a light sensor 207 at the local end thereof. The '234 Patent indicates that in other embodiments the temperature sensor may comprise other temperature measurement devices such as a thermistor or thermocouple. However, the '134 Patent does not disclose any system or method which is used to prevent RF heating from occurring in the first instance. In addition, the temperature measurement system of the '134 Patent is used to monitor changes in the 10 temperature of the inserted device, not to monitor the patients temperature.
U.S. Pat. No. 5,209,233 to Holland et al. ("the '233 Patent") discloses a system which monitors the temperature of ECG electrodes 40 mounted on a patient 8 undergoing an MRI scan (FIG. 2) to ensure that ECG electrode 40 does not undergo "localized heating" during an MRI examination and to allow monitoring of the patient's skin temperature. Although the '233 Patent does disclose impedances 64 and 82 connected in series with the RTD or thermistor 60 located at the ECG electrode to prevent excessive currents from being induced in lead 62 by either radio frequency signals or changing magnetic field gradients, the '233 Patent does not disclose the particular structure used in the temperature measurement system of the present invention in which a thermistor probe connected in the resistive mode of operation is driven by a constant current source through a first pair of leads and an instrument amplifier is connected to the temperature sensor through a second pair of leads combine to generate a signal representative of the measured temperature. In addition, the '233 Patent does not disclose another aspect of the present invention, namely, a low battery sensing circuit which provides an indication of the status of a battery used to provide power to the temperature measurement system.
It is therefore an objective of the present invention to provide an improved system for measuring temperature of a patient within an MRI machine which eliminates thermal heating of the cables and associated sensors due to RF I.sup.2 R heating. is yet another objective of the present invention to provide an improved system for measuring temperature of a patient within an MRI machine which prevents RF noise from entering the data monitoring equipment.
It is a further objective of the present invention to provide a system for measuring temperature of a patient within an MRI machine which prevents high frequency noise signals from the data monitoring equipment from causing artifacts in an MRI image.
It is a still further object of the present invention to provide a system for measuring temperature within an MRI machine which also includes a system for indicating a low battery condition in the monitoring equipment.